1. Field of the Invention
The present invention relates to a method of controlling an optical wavelength division multiplexing transmission apparatus used in an optical transmission system, and in particular to a method of controlling an optical wavelength division multiplexing transmission apparatus which achieves stable wavelength division multiplexing optical transmission by switching the control mode of an optical amplification unit provided inside the optical wavelength division multiplexing transmission apparatus in accordance with an input state of optical signals of various wavelengths.
2. Description of the Related Art
Wavelength division multiplexing (WDM) optical transmission systems which transmit a wavelength division multiplexed optical signal comprising a plurality of optical signals of different wavelengths through a single transmission path are being taken notice as a method for achieving increased capacity in optical transmission systems. Furthermore the development of WDM optical transmission systems, which utilize an optical amplification repeater transmission mode in which an optical amplifier is used as a linear repeater, is also processing rapidly. For example, systems are being investigated which are capable of achieving optical amplification repeater transmission at transmission speeds totaling 320 Gbps, by using 32 optical signal waves of transmission speed 10 Gbps and multiplexing the 32 optical signals.
A terminal apparatus appropriate for use at the transmission end of a WDM optical transmission system as described above will be equipped with, for example, an electro-optical converting apparatus for converting an electrical signal into a narrow spectrum optical signal on the wavelength grid prescribed by ITU-T, as well as an optical wavelength division multiplexing transmission apparatus for multiplexing and then transmitting the optical signals of each wavelength output from the electro-optical converting apparatus. A specific example of the construction of an optical wavelength division multiplexing transmission apparatus is shown in FIG. 8.
With the construction shown in FIG. 8, each optical signal of different wavelength output from the electro-optical converting apparatus passes through a corresponding optical attenuation unit (VAT) and is then input into an optical multiplexing unit (TWM) and multiplexed. The wavelength division multiplexed optical signal output from the optical multiplexing unit is amplified to the desired level by an optical amplification unit (TWA) and then transmitted to an external optical transmission path or the like. Furthermore, a portion of the wavelength division multiplexed optical signal output from the optical amplification unit is sent to a spectral analysis unit (SAU) where a spectral analysis is conducted and the amount of optical attenuation at each of the optical attenuation units then controlled in accordance with the results of the spectral analysis. Moreover, the optical amplification unit is subject to automatic level control (ALC) in steady state conditions, so that the level of the optical output is controlled to give a constant level set in accordance with the number of wavelengths being used.
However, with optical transmission apparatus like those described above, a problem arises in that if an increase or a decrease in the number of optical wavelengths being input occurs while the optical amplification unit is operating under ALC, then the optical amplification unit will function so as to maintain the optical output at a constant level regardless of the level of the optical input, and consequently the optical output level per single wavelength will fluctuate. As a result, the operation of repeater devices or a receiving end terminal apparatus connected to the optical wavelength division multiplexing transmission apparatus will also be affected, resulting in a potential deterioration in the transmission properties.
In order to resolve the aforementioned problem, it is possible for example, to interrupt the ALC operation of the optical amplification unit during increases or decreases in the number of wavelengths being input, and then subsequently alter the operating conditions of the optical amplification unit so that an optical output level per single wave which corresponds to the new number of wavelengths after increased or decreased is achieved. However, if the optical output level is adjusted simply by varying the gain of the optical amplification unit, then the deviation (tilt) of the gain between each different wavelength will also vary, making it difficult to maintain at a constant level the optical level of each wavelength of the amplified wavelength division multiplexed optical signal. Until now, no specific control method has been proposed which produces no fluctuation in the optical level per single wave of a wavelength division multiplexed optical signal output from an optical amplification unit, and yet is able to maintain the optical level of each wavelength at a constant level.